From reading what you wrote its plain to see that he is an addict of many years. So your parents are right.
Typically when our addicts tell us what drugs they use, how long they have been using, & how much drugs they use - they down play all of that. Its a defensive mechanism. They dont want to scare us. Especially if we are not drug users. I would bet its all worse than what he told you - possibly way worse..
I think you can figure this out by rationally looking at your life pre addict in comparison to where you are right now. Seems to me your addict has dragged you down to his level. He is an extremely manipulating person. You are being worked over big time. Its a common trait in addicts.
I also dont see what he brings to the table. He does nothing but smoke weed. You are perfect for him - you do everything. Im sure he will tell you anything he thinks you want to hear. If he can just figure out a way to successfully break you down to the point where you will no longer challenge & simply accept his drug using lifestyle. Hed be all set then.
I still have a letter from my addict. The title of the letter is My Commitment. It was written by hand not typed. A letter just filled to the brim with big promises, statements of appreciation, statements of love & commitment. Written by her while tears flowed. Ive never in all my life (including today) had anyone write me such a letter. It was incredible to read - mind blowingly beautiful. In the end, mind you, it all turned out to be pure & total BS. Every last bit of it. None of it was true.
The only reason you feel trapped in a whirlwind of emotions is because your allowing a manipulative addict to work you over.
Take some time and read some of the countless other stories on this forum. I think you will find some answers.
Please take care.